Three more quick thoughts about Casey:
1.) Is it just me, or are there crazier vice presidential picks than Bob Casey? He may not be a star in the Senate, as Eve points out. But he's popular with the people Obama is weakest among, and who, if Obama were the nominee, would be at greatest risk of defecting to McCain. (Also, don't confuse inside-the-beltway reviews with home-state appeal.)
Pennsylvania defections are a real concern for Obama given how close the state's been in recent elections. It's a state that, under any conventional electoral map, the Democratic nominee has to carry. I'd bet the idea of putting Casey on the ticket has come up in Obamaland in recent days.*
2.) Which brings me to the next point: I'm guessing the audience for this endorsement is Casey's fellow superdelegates as much as it's voters in Pennsylvania. It says to the supers: "Don't worry about white working-class defections. Bob Casey is going to help me lock down that demographic, and we're not going to have trouble holding this state." That may or may not be true, but it does send a powerful message. (Not as powerful as Obama actually doing well among white working-class voters in the primary, but still pretty powerful.)
*One obvious hitch: The pro-choice groups would probably go nuts since Casey opposes abortion. But even this may not be insurmountable given the appetite for beating McCain. And other anti-abortion pols have managed to put pro-choicers at ease of late. See, for example, the account of Colorado Governor Bill Ritter's recent election in Amy Sullivan's The Party Faithful. (Yes, that Amy Sullivan.) Still, I concede that it's a longshot.